Method of performing insertion loss estimation

ABSTRACT

The present invention provides a method and apparatus for estimating the insertion loss of a telephone line. A complex waveform is applied to each wire of the telephone line being tested. Real and imaginary components of the resultant waveform are measured at a plurality of frequencies. Insertion loss of the line is estimated from a series of single-ended voltage measurements made at a plurality of frequencies. These measurements are captured and used to estimate the insertion loss of the telephone line.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the United States national phase of InternationalApplication No. PCT/IB01/01273, filed on Jul. 16, 2001, which, in turn,claims priority to United Kingdom Application No. 0017732.9 (now GB2365253), filed on Jul. 19, 2000.

BACKGROUND OF THE INVENTION

This invention relates generally to communication networks and moreparticularly to systems for qualifying telephone lines for datatransmission. As is known in the art, public switch telephone networks,i.e., so-called plain old telephone service (POTS) lines, wereoriginally designed for voice communications, which cover a limitedfrequency bandwidth (i.e., about 4 KHz). Today, it is desired to use thesame POTS lines for data transmission. Data signals, however, generallyhave different frequency characteristics than voice signals. As aresult, a POTS line that works well transmitting voice signals might notwork well, or may not work at all, for data signals. Telephone companiesneed to know which lines are suitable, i.e., qualify, and which linesare not suitable for data transmission. Telephone companies also need toknow the reason why particular lines are unable to support datatransmissions and where such faults occur so they can determine whetherthe transmission line can be corrected.

There are problems for telephone operating companies (Telco's)attempting to qualify subscriber loops for delivery of data. One problemis strategic. Telco's are reluctant to deploy emerging technologies forthe delivery of data transmission services (e.g., ISDN or ADSL) becausethere is uncertainty in their knowledge that sufficient of thesubscriber loops are of high enough quality to make deploymenteconomically successful. This discourages early adopters because thereis significant risk in being first to deliver a technology that may notwork in their access network. If Telco's could be given a technology totake much of this risk out of initial deployment, they can secure marketshare and lead in the face of competition.

An additional problem is tactical and comes after a Telco has made adecision to deploy a particular technology. There is a need to qualify,either pro-actively or reactively, specific lines for service as thatservice is requested by subscribers or targeted by the Telco fordelivery. There are a number of factors which decrease the end to enddata transmission rate attainable on a pair of wires of a telephoneline. Some of these factors are imbalanced lines, contact faults and thelike. Given that a telephone line has no other parasitic conditions ornoise interferers, the operation of the service on the line ultimatelydepends on the overall attenuation or insertion loss of the wire pair tothe signal applied. Currently telephone companies measure insertion lossby deploying personnel to either end of the wire pair to measure theinsertion loss at different frequencies (e.g. 100 kHz, 300 kHz, etc.)through hand held instruments. This procedure is expensive, laborintensive, and time consuming. It would be desirable to have anapparatus and method for estimating the insertion loss of a line fordata transmission services, and further that the method and apparatus besimple to implement, efficient, and not require the deployment ofpersonnel to remote locations.

SUMMARY OF THE INVENTION

With the foregoing background in mind, it is an object of the presentinvention to provide a method and apparatus for accurately estimatingthe insertion loss of a telephone line. Insertion loss of the line isestimated from a series of single-ended voltage measurements made at aplurality of frequencies. A complex waveform having multiple frequenciesis applied to the telephone line being tested. Real and imaginarycomponents of the resultant waveform are measured. These measurementsare captured and used to accurately estimate the insertion loss of thetelephone line at one or more frequencies. From the estimated insertionloss a determination of the data service supportable by the line can bemade.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood by reference to the followingmore detailed description and accompanying drawings in which:

FIG. 1 is a block diagram of the remote measuring unit coupled to a lineunder test;

FIG. 2 is a graph showing the estimated insertion loss for a telephoneline; and

FIG. 3 is a flow chart of a method of measuring insertion loss of atelephone line.

DETAILED DESCRIPTION

The telephone network was originally designed for voice communication.Voice communication covers a limited frequency bandwidth. In some cases,telephone lines were optimized for signals with this frequency range(approximately 4 kHz). Even where the lines were not optimized for voicesignals, there was no incentive to make the lines operate at otherfrequencies and often they did not. Now, it is desired to use those samelines to carry data signals. The data signals generally have differentfrequency characteristics than the voice signals. As a result, a linethat works very well transmitting voice signals might not work well orat all for data signals. Phone companies need to know which lines willwork for data signals in order to offer data services to customers ofthose lines.

The present invention provides a method and apparatus for estimating theinsertion loss of one or more telephone lines. The estimate isindependent of cable fill (lines within a bundle which are actuallyconnected to a line circuit) and represents the loss between the wirepairs (known as A and B or Tip and Ring) along with any associatedcoupling to adjacent wire pairs. For any such estimate to be practicalthe termination device on the line does not have any effect on theestimate. Additionally, the cable gauges and wire material is accountedfor.

An A/B pair in a bundle of wires is capacitively coupled between A and Band is also capacitively coupled to surrounding adjacent wires. Thecombination of such capacitive coupling presents a mutual capacitancevalue. For any given distance, such mutual capacitance values across thepair are coupled resistively through the distributed resistance of thewire along the length of the pair.

Thus, for any potential difference across the pair the capacitive mutualcoupling tends to conduct AC and more so as frequency increases. This isthen tempered by the distributed resistance in the pair tending toresist current flow but is frequency independent.

Cable is procured on the basis of a fixed value of mutual capacitance(e.g. 54 nF per km), as far as is possible cable is procured at thisvalue regardless of cable gauge. Essentially because this mutualcapacitance value is fixed or varies by only a small to moderate amountfor different cables, the load presented to an AC drive varies bylength, (the total sum of the capacitive coupling for a pair) and bycable resistance. The cable resistance changes by large amountsdepending on the cable gauge and material, e.g. smaller gauges are muchmore resistive preventing current flow, aluminum is more resistive thatcopper and therefore prevents current flow more than copper for the samecable gauge.

For a fixed real length of cable, for example one km, the mutualcapacitance contribution would remain fixed (e.g. at 54 nF) but theimpedance measured at one end would vary with higher impedance beingseen through small gauge cables than through large gauge cables.Similarly if the cable were 2 km long the mutual capacitive couplingwould be 108 nF but again cable gauge affects the impedance beingpresented to the measurement system.

AC current flow to earth is dependent upon cable fill (i.e. linesconnected to a line circuit present an earth path). Adjacent wire pairsaffect overall loss of data signals whether they are part of the fill ornot. In any estimation of insertion loss it is important to either:

-   -   1) greatly reduce any current flowing to earth from the pair        under test caused by a variable unknown fill level, or,    -   2) by other means negate the effect of this variable fill.        This disclosure gives two means by which the estimation of        insertion loss can be made independent of cable fill. These        means are as follows:

In order to reduce current flowing from the pair under test to theadjacent pairs the complex drive voltage can be driven onto either legof the pair but with a phase difference of 180 degrees (later referredto as drive/-drive) between the two wires to maximize the potentialdifference between the legs of the pair. Additionally the signalgeneration can be isolated from earth in the test head to eliminate areturn current path.

To negate the effect of the cable fill even when the signal being drivenonto the pair in common mode (later referred to as drive/drive) it ispossible to eliminate much of the effect of cable fill by adjusting theestimate of insertion loss by examining the high frequency behavior ofthe complex waveform applied.

Referring to FIG. 1, a block diagram of the present invention 10 isshown coupled to a selected telephone line 150. The test unit 10includes an interconnection element 15, and a signal source andmeasurement unit 11. The test unit 110 measures the wires 30 and 40 of aselected telephone line 50.

The test unit 10 comprises a hardware/software system specificallydeveloped for estimating insertion loss of a telephone line. Thesoftware includes commands for applying a complex waveform having aplurality of frequencies to a telephone line (for example the complexwaveform may have approximately 45 different frequencies), commands fordirecting the test unit to measure the voltages of the telephone line,and commands for calculating the estimated insertion loss of thetelephone line from the measurements. Other functions such as predictingthe line length may also be provided by the test unit. The test unit 10also includes storage for storing the values of the measured variablesas well as storing the commands associated with estimating the insertionloss of the selected telephone line.

In a first embodiment, the A/B wires 30 and 40 of the selected telephoneline 50 are placed in communication with the signal source. The signalsource is applied differentially (drive/-drive) to the A/B pair,optionally the test head instrumentation can be isolated from earthwhich will give more accurate estimations of loss.

Modems, either analog or ADSL, apply their tones to the phone linedifferentially and without reference to earth. A good method ofassessing a lines performance to such tones is by use of fieldinstrumentation to assess the insertion loss of the line. Such equipmentcouples the tone transmitter and tone receiver to the line viatransformers giving isolation from earth. In any event, field equipmentdoes not enjoy ready access to earth for reference anyway. In summary,both modems and the primary method of assessing line performance have noreference to earth; therefore currents do not flow to earth whentransmitting modem tones or when measuring insertion loss by known dualended techniques. Thus, a loss based estimate for a line may be obtainedthat mimics dual ended insertion loss measurements provided that thecurrent flowing in the load presented by the line is mainly between theA and B wires of the line and that no current or very little current isflowing to earth.

The test unit 10 applies a complex waveform on each of the A/B wires 30and 40 in what is known as a drive/-drive configuration. Measurements ofboth real and imaginary voltage components over a number of frequenciesfrom approximately 100 Hz to approximately 20 kHz are made. The voltagemeasurements at the lower frequency could be affected to a small extentby the telephone-terminating device; this effect is reduced by usingdrive voltages of 500 mV or less. Alternately a fixed reference valuemay be substituted for frequencies at around 100 Hz, the value basedupon the output of the signal source at that frequency. The next four ormore frequencies having measurable real and imaginary voltage componentsare used to describe a loss-based trend.

The test unit 10 utilizes the measurement unit to measure real andimaginary components of the resultant waveform between the wires 130 and140. These measurements are done at a plurality of frequencies. Forexample, the measurements are made at five frequencies. The fivefrequencies are between 0 and 20 kHz, and are designated as f₁, f₂, f₃,f₄, and f₅. The voltage measurements include real components of thevoltages, designated V_(Real), and imaginary components of the voltages,designated V_(Imag). Thus the real and imaginary voltage measurements ateach frequency are designated V_(Real)@f₁, V_(Imag)@f₁, V_(Real)@f₂,V_(Imag)@f₂, . . . V_(Real)@f₅, V_(Imag)@f₅. Test unit 10 utilizes thesevoltage measurements to estimate the insertion loss of the selectedtelephone line according to the formula:k.log₁₀(((V_(Real)@f₂)²+(V_(Imag)@f₂)²+(V_(Real)@f₃)²+(V_(Imag)@f₃)²+(V_(Real)@f₄)²+(V_(Imag)@f₄)²+(V_(Real)@f₅)²+(V_(Imag)@f₅)²)/(X))where k is a constant related to a data access rate and X is a referencevalue.

The constant k is a value, which varies in accordance with the desiredlevel of service the line is being qualified for. For example at a firstlevel of service k would be a first constant such as 84, while at asecond level of service k would have a different value such as 130.Reference value X is either the value:4×(|V_(open)|)²or4×((V_(Real)@f1)²+(V_(Imag)@f1)²)where V_(open) is the open line output of the test unit.

The number arrived at by the method comprises the estimated insertionloss at 300 kHz in decibels. The above-described method utilizes thedifferential drive/-drive to remove or minimize the effects of cablefill, which would otherwise affect the estimation of the insertion loss.

The same measurements and formulae described above could also be used ina configuration where there is no isolation from ground by utilizing adrive/drive configuration. In such a configuration the complex waveformis applied common mode to the A/B wires. The complex waveform is appliedto both of the A/B wires With such an arrangement, the current flowsfrom the A/B wires to the adjacent pairs a proportion of the currentthen flows to ground from those pairs connected to line circuits.

A similar method of estimating the insertion loss of a telephone linecan be achieved as described below in an arrangement having adrive/drive configuration.

Measurements of real and imaginary voltage components over a number offrequencies from approximately 100 Hz through approximately 20 kHz aremade. The sum of the squares of these values (P_((f))) provides a valuethat is proportional to the power output of the test unit across theload (line under test).

If current is allowed to flow to earth the load presented to the testunit has all current flowing to earth, therefore the impedance presentedby such a load is now dependent on the current path to earth which inturn is a factor of cable fill. Although this should give similarresults for uniformly ‘filled’ cable this will in some instances give avery poor estimation of insertion loss, e.g. no fill in the cable wouldgive a much smaller impedance and therefore an incorrect value for loss.

An additional method of calculating insertion loss involves utilizingthe following formulas:Gradient=k.log₁₀(((V _(Real) @f ₂)²+(V _(Imag) @f ₂)²+(V _(Real) @f₃)²+(V _(Imag) @f ₃)²+(V _(Real) @f ₄)²+(V _(Imag) @f ₄)²+(V _(Real) @f₅)²+(V _(Imag) @f ₅)²)/(X))where k is a constant related to a data access rate and X is a referencevalue.Fill=Σ_(f7) ^(f22)norm(i)Where: norm(f2)=(Vreal)²+(Vimag)² for frequency 2, etc . . .Insertion Loss=scale factor*((gr ratio*Gradient)+(Gradient*Fill*fillfactor))

-   -   Where: gr ratio is a value between approximately 0.1 and 50 Fill        factor is a value between approximately 3 and 300 Scale factor        is a value between 5 and 80

The fill value accounts for the currents to ground, thus providing aninsertion loss estimate that is comparable to the drive/-drivetechnique. This method utilizes a drive/drive configuration in anon-islolated from ground arrangement. In a particular embodiment thevalue of the gr ratio is 0.36085, the value of the fill factor is160.1128 and the scale factor is 52. The above calculation yields anumber representing the insertion loss of the cable in dB at 300 kHz.

The calculated insertion loss obtained by any of the above-describedmethods is compared to a threshold (for example 41 dB). If thecalculated insertion loss is greater than or equal to the threshold,than service cannot be deployed. On the other hand, if the calculatedinsertion loss is less than the threshold, then service can be deployedon the line.

In order to provide accurate, reproducible and reliable estimations fromall of the above-described methods, additional conditions should be met.The output level and impedance of the test unit applied for thesemeasurements should be constant, stable and repeatable from test totest. The output level of test unit used for measurement should be low.The line under test may include a telephone across the wires, keepingthe output level low ensures that the telephone device remains at veryhigh impedance. It is preferable to use frequencies of a few hundred toa few thousand Hz for such measurements, at such frequencies themetallic access of most host switches through which the line is accessedhas a near linear response. The effect of the telephone device on theestimated insertion loss is kept to a minimum by application of signalsof low amplitude (<500 mV) the line terminating device e.g. telephonetends to an even higher impedance at higher frequencies.

As mentioned before, the mutual cable capacitance per unit length tendsto be fixed, unlike capacitance to earth which is variable and dependentupon cable fill. Accordingly, a method which accurately determines themutual capacitive reactance is able to measure line length veryaccurately, and in the presence of telephone devices provided thatfrequencies of 800 Hz or more are used or particular.

One of the advantages of the present invention is that it has theability to factor in both the real (resistive) and imaginary(capacitive) components of impedance for a line. Separately analyzed,the real component contribution is due to cable resistance, and theimaginary contribution due to capacitive reactance. However since onequantity is opposing the other to present such an impedance, frequencyselection at which measurements are made is quite important. At muchhigher frequencies the capacitive reactance dominates, therefore therewill be different cable makeups that would return the same value for theinsertion loss estimation. Similarly, for very long lengths of line, thecapacitive reactance will dominate and there is a folding back of theinsertion loss estimation.

Referring now to FIG. 2, a graph 60 of the estimated insertion loss of atelephone line as determined by the present invention is shown. Thehorizontal axis 62 represents the line loss for the telephone line beingtested. The vertical axis 64 represents the line length of the telephoneline being tested. Also shown is an insertion loss estimate 70 asderived by way of the presently disclosed method. The determination ofthe insertion loss estimation includes a constant k (80). k is dependenton the data access rate, with a different value of k for a respectivedata rate. For a given data rate of service (for example 2 Mb/sec DSLaccess) there is a Go/No Go threshold 90. The threshold 90 is related tothe constant k. An insertion loss above this threshold indicates thatthe selected telephone line cannot support the desired data access rate.An insertion loss below threshold 90 indicates that the selectedtelephone line will support the desired data access rate.

Referring now to FIG. 3, a flowchart showing a method 200 of estimatingthe insertion loss of a subscriber line is shown. The first step 210 isto select a telephone line to measure. A single line is measured, thoughit is preferable to measure multiple lines, one after the other.

The next step 220, is to determine the configuration to be used. Oneconfiguration requires isolation of the tip and ring or A and B wires ofthe selected telephone line from earth ground. This may be accomplishedin number of ways, such as by using one or more an isolationtransformers between the test unit and the wire pair of the selectedline. The test unit itself may be isolated from earth ground and thephone line coupled directly to the test unit. The alternateconfiguration (224) is to reference the complex waveform to ground. Oncethe grounding configuration is determined (isolated or non-isolated) thenext step is to determine the drive configuration. A drive/driveconfiguration comprises driving the same complex waveform on each of thelines. A drive/-drive configuration comprises driving a first complexwaveform on one line and driving a second complex waveform on the otherline, wherein the second complex waveform comprises a waveform which isapproximately 180° out of phase with the first complex waveform, asrecited in step 226.

The following step 230 injects a complex waveform into the wires of theselected phone line. At step 240 the voltage between the wires of theselected telephone line is measured and recorded. This measurement mayoccur at a plurality of frequencies. Both real and imaginary componentsof the voltage waveform are measured at the plurality of frequencies.

At step 250 the insertion loss of the phone line is estimated from themeasurements. The insertion loss of the selected telephone line iscalculated according to the formulae, which correspond to theconfigurations being used.

Thus, from the above described methods and apparatus, an insertion lossestimation for a telephone line is obtained. A complex signal isprovided and a series of voltage measurements are made and recorded. Theinsertion loss for the line is estimated from the voltage measurements.

Having described preferred embodiments of the invention it will nowbecome apparent to those of ordinary skill in the art that otherembodiments incorporating these concepts may be used. Additionally, thesoftware included as part of the invention may be embodied in a computerprogram product that includes a computer useable medium. For example,such a computer usable medium can include a readable memory device, suchas a hard drive device, a CD-ROM, a DVD-ROM, or a computer diskette,having computer readable program code segments stored thereon.Accordingly, it is submitted that that the invention should not belimited to the described embodiments but rather should be limited onlyby the spirit and scope of the appended claims.

1. A method of estimating a characteristic of a subscriber line, thesubscriber line including a first wire and a second wire, the methodcomprising the steps of: applying a first complex waveform to a firstwire of the subscriber line; applying a second complex waveform to asecond wire of the subscriber line; obtaining voltage measurementsbetween said first wire and said second wire of the subscriber line; andestimating insertion loss of said line from said voltage measurements,including adjusting the estimate of insertion loss based on the highfrequency behavior of the complex waveform, wherein adjusting theestimate of insertion loss negates the effect of variable cable fill. 2.The method of claim 1 wherein said second complex waveform comprisessaid first complex waveform.
 3. The method of claim 1 wherein saidsecond complex waveform comprises a waveform approximately 180° out ofphase with said first complex waveform.
 4. The method of claim 1 furtherincluding a ground configuration selected from the group consisting ofan isolated ground configuration and a non-isolated groundconfiguration.
 5. The method of claim 1 wherein said step of obtainingvoltage measurements comprises obtaining real and imaginary voltagemeasurements, designated V_(Real) and V_(Imag) respectively.
 6. Themethod of claim 5 wherein said step of obtaining voltage measurementscomprises obtaining voltage measurements at five or more differentfrequencies, said frequencies designated f₁, f₂, f₃, f₄, and f₅.
 7. Themethod of claim 1 wherein said step of estimating an insertion loss isperformed according to the formula:k.log₁₀(((V_(Real)@f₂)²+(V_(Imag)@f₂)²+(V_(Real)@f₃)²+(V_(Imag)@f₃)²+(V_(Real)@f₄)²+(V_(Imag)@f₄)²+(V_(Real)@f₅)²+(V_(Imag)@f₅)²)/(X));wherein k is a constant and X is a reference value.
 8. The method ofclaim 7 wherein X is selected from the group consisting of(4×(V_(Real)@f₁)²+(V_(Imag)@f₁)²) and (4×(|V_(Open)|)²) wherein V_(open)comprises the open line output measurement.
 9. The method of claim 7wherein k is selected from the group consisting of 84 and
 130. 10. Themethod of claim 1 wherein said first complex waveform includes aplurality of frequencies between 0 and 20 kHz.
 11. The method of claim 1wherein said step of obtaining comprises performing a single endedmeasurement.
 12. The method of claim 1 wherein said step of estimatingan insertion loss is performed according to the formula:Insertion Loss=scale factor*((gr ratio*Gradient)+(Gradient*Fill*fillfactor)).
 13. The method of claim 12 wherein the variable Gradient isdefined as:Gradient=k.log₁₀(((V _(Real) @f ₂)²+(V _(Imag)@f₂)²+(V _(Real) @f ₃)²+(V_(Imag) @f ₃)²+(V _(Real) @f ₄)²+(V _(Imag) @f ₄)²+(V _(Real) @f ₅)²+(V_(Imag) @f ₅)²)/(X)).
 14. The method of claim 12 wherein the variableFill is defined as:Fill=Σ_(f7) ^(f22)norm(i).
 15. The method of claim 14 wherein norm(i) isdefined as:norm(fi)=(V _(Real))²+(V _(Imag))² for frequency i.
 16. The method ofclaim 12 wherein the variable gr ratio is between approximately 0.1 and50.
 17. The method of claim 12 wherein the variable fill factor isbetween approximately 3 and
 300. 18. The method of claim 12 wherein thevariable scale factor is between approximately 5 and
 80. 19. A computerprogram product for estimating insertion loss of a line, the computerprogram product comprising a computer usable medium having computerreadable code thereon, including program code comprising: instructionsfor causing a test unit to perform at least one of selecting a groundconfiguration, applying a first complex waveform to a first wire of theline, applying a second complex waveform to the second wire of the line,obtaining voltage measurements between said first wire and said secondwire of the line, and estimating insertion loss of said line from saidmeasurements, including adjusting the estimate of insertion loss basedon the high frequency behavior of the complex waveform, whereinadjusting the estimate of insertion loss negates the effect of variablecable fill.
 20. The computer program product of claim 19 wherein saidsecond complex waveform comprises said first complex waveform.
 21. Thecomputer program product of claim 19 wherein said second complexwaveform comprises a waveform approximately 180° out of phase with saidfirst complex waveform.
 22. The computer program product of claim 19wherein said line has a ground configuration selected from the groupconsisting of an isolated ground configuration and a non-isolated groundconfiguration.
 23. The computer program product of claim 19 wherein saidinstructions cause said test unit to obtain voltage measurements at fiveor more different frequencies, said frequencies designated f₁, f₂, f₃,f₄, and f₅.
 24. The computer program product of claim 19 wherein saidinstructions cause said test unit to obtain real and imaginary voltagemeasurements, designated V_(Real) and V_(Imag) respectively.
 25. Thecomputer program product of claim 19 wherein said insertion loss isestimated according to the formula:k.log₁₀(((V_(Real)@f₂)²+(V_(Imag)@f₂)²+(V_(Real)@f₃)²+(V_(Imag)@f₃)²+(V_(Real)@f₄)²+(V_(Imag)@f₄)²+(V_(Real)@f₅)²+(V_(Imag)@f₅)²)/(X));wherein k is a constant and X is a reference value.
 26. The computerprogram product of claim 25 wherein X is selected from the groupconsisting of(4×(V_(Real)@f₁)²+(V_(Imag)@f₁)²) and (4×(|V_(Open)|)²) wherein V_(Open)comprises the open line output measurement.
 27. The computer programproduct of claim 25 wherein k is selected from the group consisting of84 and
 130. 28. The computer program product of claim 19 wherein saidfirst complex waveform includes a plurality of frequencies between 0 and20 kHz.
 29. The computer program product of claim 19 wherein saidmeasurements are single ended measurements.
 30. The computer programproduct of claim 19 wherein said insertion loss is estimated accordingto the formula:Insertion Loss=scale factor*((gr ratio*Gradient)+(Gradient*Fill*fillfactor)).
 31. The computer program product of claim 30 wherein thevariable Gradient is defined as:Gradient=k.log₁₀(((V _(Real) @f ₂)²+(V _(Imag) @f ₂)²+(V _(Real) @f₃)²+(V _(Imag) @f ₃)²+(V _(Real) @f ₄)²+(V _(Imag) @f ₄)²+(V _(Real) @f₅)²+(V _(Imag) @f ₅)²)/(X)).
 32. The computer program product of claim30 wherein the variable Fill is defined as:Fill=Σ_(f7) ^(f22)norm(i).
 33. The computer program product of claim 32wherein norm(i) is defined as:norm(fi)=(V _(Real))²+(V _(Imag))² for frequency i.
 34. The computerprogram product of claim 30 wherein the variable gr ratio is betweenapproximately 0.1 and
 50. 35. The computer program product of claim 30wherein the variable fill factor is between approximately 3 and
 300. 36.The computer program product of claim 30 wherein the variable scalefactor is between approximately 5 and approximately
 80. 37. Apparatusfor estimating insertion loss of a subscriber line, the subscriber lineincluding a first wire and a second wire, the apparatus comprising:first applying means for applying a first complex waveform relative to aselected ground to a first wire of the subscriber line; second applyingmeans for applying a second complex waveform relative to the selectedground to a second wire of the subscriber line; obtaining means forobtaining voltage measurements between said first wire and said secondwire of the subscriber line; and estimating means for estimatinginsertion loss of said line from said voltage measurements and adjustingthe estimate of insertion loss based on the high frequency behavior ofthe complex waveform, wherein adjusting the estimate of insertion lossnegates the effect of variable cable fill.
 38. The apparatus of claim 37wherein said first and second applying means are adapted to apply thesame complex waveform relative to the selected ground to the first andsecond wires of the subscriber line respectively.
 39. The apparatus ofclaim 37 wherein said second applying means is adapted to apply saidsecond complex waveform comprising a waveform of approximately 180° outof phase with said first complex waveform.
 40. The apparatus of claim 37wherein said selected ground is either an isolated ground configurationor a non-isolated ground configuration.
 41. The apparatus of claim 37wherein said obtaining means is adapted to obtain real and imaginaryvoltage measurements, designated V_(Real) and V_(Imag) respectively. 42.The apparatus of claim 41 wherein said obtaining means is adapted toobtain voltage measurements at five or more different frequencies. 43.The apparatus of claim 37 wherein said estimating means is adapted toestimate an insertion loss using the formula:k.log₁₀(((V_(Real)@f₂)²+(V_(Imag)@f₂)²+(V_(Real)@f₃)²+(V_(Imag)@f₃)²+(V_(Real)@f₄)²+(V_(Imag)@f₄)²+(V_(Real)@f₅)²+(V_(Imag)@f₅)²)/(X));wherein k is a constant and X is a reference value.
 44. The apparatus ofclaim 43 wherein X is selected from the group consisting of:(4×(V_(Real)@f₁)²+(V_(Imag)@f₁)²) and (4×(|V_(Open)|)²) wherein V_(Open)comprises the open line output measurement.
 45. The apparatus of claim43 wherein k is selected from the group consisting of 84 and
 130. 46.The apparatus of claim 37 whence the first complex waveform includes aplurality of frequencies between 0 and 20 kHz.
 47. The apparatus ofclaim 37 wherein said obtaining means is adapted to perform a singleended measurement.
 48. The apparatus of claim 37 wherein said estimatingmeans is adapted to estimate a length of said line.
 49. The apparatus ofclaim 37 wherein said estimating means is adapted to estimate insertionloss using the formula:Insertion Loss=scale factor*((gr ratio*Gradient)+(Gradient*Fill*fillfactor)).
 50. The apparatus of claim 49 wherein the variable Gradient isdefined as:k.log₁₀(((V_(Real)@f₂)²+(V_(Imag)@f₂)²+(V_(Real)@f₃)²+(V_(Imag)@f₃)²+(V_(Real)@f₄)²+(V_(Imag)@f₄)²+(V_(Real)@f₅)²+(V_(Imag)@f₅)²)/(X)).51. The apparatus of claim 49 wherein the variable Fill is defined as:Fill=Σ_(f7) ^(f22)norm(i).
 52. The apparatus of claim 51 wherein norm(i)is defined as:norm(fi)=(V _(Real))²+(V _(Imag))² for frequency i.
 53. The apparatus ofclaim 49 wherein the variable gr ratio is between approximately 0.1 and50.
 54. The apparatus of claim 49 wherein the variable fill factor isbetween approximately 3 and
 300. 55. The apparatus of claim 49 whereinthe variable scale factor is between approximately 5 and 80.